xv CASTOREI3ST AND ITS USES 469 forefathers. The Rev. Edward Topsell observed that " for giving great ease unto the gowt the skinnes of beavers burned with drie oynions " are excellent. Castorein as a drug, if not in actual use, has quite recently been a part of the pharmacopoeia. It is derived from the anal glands common to this and other Kodents, and indeed many other mammals. A large extinct form of Beaver is Trogontherium,1 found in the " Forest-bed " of Cromer. The skull is about one-fourth longer than that of Oast or. It has a less inflated bulla, and slightly more pronounced postorbital processes than Castor. The third molar (fourth grinding tooth) is relatively larger than in Castor, and has a rather more folded crown. The foramen magnum is more triangular. Fam. 4. Hapiodontidae.—A separate family seems to be re- quired for the genus Haplodon, whose characters will therefore be merged with those of its family. It is to be distinguished from most other Squirrel-like creatures by the fact that there is no post- orbital process to the frontal. The molar teeth are five in the upper and four in the lower jaw. The Sewellel, J5T. ritfus, like the other species of the genus (JJ. mcgor\ is found in North America west of the Hocky Mountains. It has the habit of the Prairie-marmot, and has a short tail, only moderately long ears, and five-toed feet. Tullberg is of opinion that this animal nearly represents the ancestral form of the Squirrel tribe. SECTION 2. MYOMORPHA. This subdivision of the Hodents contains, according to Mr. Thomas's recent estimate/ no less than 102 genera. It is there- fore obviously impossible to do more than refer to some of the more interesting of these. This group is again divided into the following families:— (1) Grliridae, including the Dormice. (2) Muridae, the Itats, Mice, Gerbiiles, Australian Water-rats, Hamster. (3) Bathyergidae, Cape Mole, etc. (4) Spalacidae, IBamboo Hats. 1 E. T. Newton, Trans, Zool, Soe. xlii. 1892, p. 165. * JVoc. Zool. Soc* 1896, p. 1O16.